The hashtags used on Google+ work in two ways:
You can click on them (when blue) and they will take you straight to the search, showing posts with the hashtag in question. People who search for a certain topic can do so with or without the # and a keyword: You get slightly different results with either method, I've noticed.
Using hashes is a good way to get your posts noticed by the public in search, not just in photo themes but on any topic, works also in Communities.
Via Jaana Nyström, Gerrit Bes